Activation-dependent properties of pregnenolone sulfate inhibition of GABAA receptor-mediated current

Divergent mechanisms of steroid inhibition in the human ρ1 GABAA receptor

ρ-type γ-aminobutyric acid-A (GABAA) receptors are widely distributed in the retina and brain, and are potential drug targets for the treatment of visual, sleep and cognitive disorders. Endogenous neuroactive steroids including β-estradiol and pregnenolone sulfate negatively modulate the function of ρ1 GABAA receptors, but their inhibitory mechanisms are not clear. By combining five cryo-EM structures with electrophysiology and molecular dynamics simulations, we characterize binding sites and negative modulation mechanisms of β-estradiol and pregnenolone sulfate at the human ρ1 GABAA receptor. β-estradiol binds in a pocket at the interface between extracellular and transmembrane domains, apparently specific to the ρ subfamily, and disturbs allosteric conformational transitions linking GABA binding to pore opening. In contrast, pregnenolone sulfate binds inside the pore to block ion permeation, with a preference for activated structures. These results illuminate contrasting mechanisms of ρ1 inhibition by two different neuroactive steroids, with potential implications for subtype-specific gating and pharmacological design.

Forty Years Searching for Neurosteroid Binding Sites on GABAA Receptors

Brain inhibition is a vital process for controlling and sculpting the excitability of the central nervous system in healthy individuals. This level of control is provided over several timescales and involves the neurotransmitter GABA acting at inhibitory synapses to: rapidly inhibit neurons by activating the GABAA receptor; over a slower timescale, to tonically activate extrasynaptic GABAA receptors to provide a low level of background inhibition; and finally, to activate G-protein coupled GABAB receptors to control transmitter release by inhibiting presynaptic Ca2+ channels whilst providing postsynaptic inhibition via K+ channel activation. From this plethora of roles for GABA and its receptors, the GABAA receptor isoform is of major interest due to its dynamic functional plasticity, which in part, is due to being targeted by modulatory brain neurosteroids derived from sex and stress hormones. This family of neurosteroids can, depending on their structure, potentiate, activate and also inhibit the activity of GABAA receptors to affect brain inhibition. This review tracks the methods that have been deployed in probing GABAA receptors, and charts the sterling efforts made by several groups to locate the key neurosteroid binding sites that affect these important receptors. Increasing our knowledge of these binding sites will greatly facilitate our understanding of the physiological roles of neurosteroids and will help to advance their use as novel therapeutics to combat debilitating brain diseases.

Structural insights into opposing actions of neurosteroids on GABAA receptors

γ-Aminobutyric acid type A (GABAA) receptors mediate fast inhibitory signaling in the brain and are targets of numerous drugs and endogenous neurosteroids. A subset of neurosteroids are GABAA receptor positive allosteric modulators; one of these, allopregnanolone, is the only drug approved specifically for treating postpartum depression. There is a consensus emerging from structural, physiological and photolabeling studies as to where positive modulators bind, but how they potentiate GABA activation remains unclear. Other neurosteroids are negative modulators of GABAA receptors, but their binding sites remain debated. Here we present structures of a synaptic GABAA receptor bound to allopregnanolone and two inhibitory sulfated neurosteroids. Allopregnanolone binds at the receptor-bilayer interface, in the consensus potentiator site. In contrast, inhibitory neurosteroids bind in the pore. MD simulations and electrophysiology support a mechanism by which allopregnanolone potentiates channel activity and suggest the dominant mechanism for sulfated neurosteroid inhibition is through pore block.

Neurosteroid enantiomers as potentially novel neurotherapeutics

Endogenous neurosteroids and synthetic neuroactive steroids (NAS) are important targets for therapeutic development in neuropsychiatric disorders. These steroids modulate major signaling systems in the brain and intracellular processes including inflammation, cellular stress and autophagy. In this review, we describe studies performed using unnatural enantiomers of key neurosteroids, which are physiochemically identical to their natural counterparts except for rotation of polarized light. These studies led to insights in how NAS interact with receptors, ion channels and intracellular sites of action. Certain effects of NAS show high enantioselectivity, consistent with actions in chiral environments and likely direct interactions with signaling proteins. Other effects show no enantioselectivity and even reverse enantioselectivity. The spectrum of effects of NAS enantiomers raises the possibility that these agents, once considered only as tools for preclinical studies, have therapeutic potential that complements and in some cases may exceed their natural counterparts. Here we review studies of NAS enantiomers from the perspective of their potential development as novel neurotherapeutics.

The Sulfated Steroids Pregnenolone Sulfate and Dehydroepiandrosterone Sulfate Inhibit the α1β3γ2L GABAA Receptor by Stabilizing a Novel Nonconducting State

The GABA_A receptor is inhibited by the endogenous sulfated steroids pregnenolone sulfate (PS) and dehydroepiandrosterone sulfate (DHEAS). It has been proposed in previous work that these steroids act by enhancing desensitization of the receptor. Here, we have investigated the modulatory effects of the steroids on the human α1β3γ2L GABA_A receptor. Using electrophysiology and quantitative model-based data analysis, we show that exposure to the steroid promotes occupancy of a nonconducting state that retains high affinity to the transmitter but whose properties differ from those of the classic, transmitter-induced desensitized state. From the analysis of the inhibitory actions of two combined steroids, we infer that PS and DHEAS act through shared or overlapping binding sites. SIGNIFICANCE STATEMENT: Previous work has proposed that sulfated neurosteroids inhibit the GABA_A receptor by enhancing the rate of entry into the desensitized state. This study shows that the inhibitory steroids pregnenolone sulfate and dehydroepiandrosterone sulfate act through a common interaction site by stabilizing a distinct nonconducting state.

Pharmacological characterization of a novel negative allosteric modulator of NMDA receptors, UBP792

N-methyl-d-aspartate (NMDA) receptors (NMDARs) are a subtype of ionotropic glutamate receptor with important roles in CNS function. Since excessive NMDAR activity can lead to neuronal cell death and epilepsy, there is interest in developing NMDAR negative allosteric modulators (NAMs) as neuroprotective agents. In this study, we characterize the inhibitory properties of a novel NMDAR antagonist, UBP792. This compound displays partial subtype-selectivity by having a varied maximal inhibition of GluN2A-, GluN2B-, GluN2C-, and GluN2D-containing receptors (52%, 70%, 87%, 89%, respectively) with IC50s 4-10 μM. UBP792 inhibited NMDAR responses by reducing l-glutamate and glycine potencies and efficacies. Consistent with non-competitive inhibition, increasing agonist concentrations 30-fold did not reduce UBP792 potency. UBP792 inhibition was also not competitive with the structurally-related positive allosteric modulator (PAM) UBP684. UBP792 activity was voltage-independent, unaffected by GluN1's exon-5, and reduced at low pH (except for GluN1/GluN2A receptors which were more sensitive at acidic pH). UBP792 binding appeared independent of agonist binding and may be entering the plasma membrane to gain access to its binding site. Inhibition by UBP792 is reduced when the ligand-binding domain (LBD) of the GluN2 subunit, but not that of the GluN1 subunit, is cross-linked in the closed-cleft, activated conformation. Thus, UBP792 may be inhibiting by stabilizing an open GluN2-LBD cleft associated with channel inactivation or by stabilizing downstream closed channel conformations allosterically-coupled to the GluN2-LBD. These findings further expand the repertoire displayed by NMDAR NAMs thus expanding the opportunities for developing NMDAR modulators with the most appropriate selectivity and physiological actions for specific therapeutic indications.

Biosynthesis and signalling functions of central and peripheral nervous system neurosteroids in health and disease

Neurosteroids are steroid hormones synthesised de novo in the brain and peripheral nervous tissues. In contrast to adrenal steroid hormones that act on intracellular nuclear receptors, neurosteroids directly modulate plasma membrane ion channels and regulate intracellular signalling. This review provides an overview of the work that led to the discovery of neurosteroids, our current understanding of their intracellular biosynthetic machinery, and their roles in regulating the development and function of nervous tissue. Neurosteroids mediate signalling in the brain via multiple mechanisms. Here, we describe in detail their effects on GABA (inhibitory) and NMDA (excitatory) receptors, two signalling pathways of opposing function. Furthermore, emerging evidence points to altered neurosteroid function and signalling in neurological disease. This review focuses on neurodegenerative diseases associated with altered neurosteroid metabolism, mainly Niemann-Pick type C, multiple sclerosis and Alzheimer disease. Finally, we summarise the use of natural and synthetic neurosteroids as current and emerging therapeutics alongside their potential use as disease biomarkers.

A neuroactive steroid with a therapeutically interesting constellation of actions at GABAA and NMDA receptors

Neuroactive steroids are an ascendant class of treatment for neuropsychiatric illness. Effects on ligand-gated neurotransmitter receptors appear to be a major mechanism of action. Here we describe a neuroactive steroid with a unique constellation of receptor actions. MQ-221 is a sulfated, 3β-hydroxy neurosteroid analogue that inhibits NMDAR function but also potentiates GABAAR function, thereby exhibiting unusual but potentially clinically desirable effects. Although the compound also exhibited features of other sulfated steroids, namely activation-dependent inhibition of GABAAR function, net potentiation dominated under physiological conditions. Potentiation of GABAAR function was distinct from the mechanism governing potentiation by anesthetic neurosteroids. Inhibition of NMDAR function showed weaker channel activation dependence than pregnanolone sulfate (3α5βPS). MQ-221 was unique among four stereoisomers explored in the pattern of effects at GABAA and NMDARs. Taken together, MQ-221 may represent a new class of compound with unique psychoactive effects and beneficial prospects for treating neuropsychiatric disorders.

Progesterone modulates neuronal excitability bidirectionally

Progesterone acts on neurons directly by activating its receptor and through metabolic conversion to neurosteroids. There is emerging evidence that progesterone exerts excitatory effects by activating its cognate receptors (progesterone receptors, PRs) through enhanced expression of α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid receptors (AMPARs). Progesterone metabolite 5α,3α-tetrahydro-progesterone (allopregnanolone, THP) mediates its anxiolytic and sedative actions through the potentiation of synaptic and extrasynaptic γ-aminobutyric acid type-A receptors (GABAARs). Here, we review progesterone's neuromodulatory actions exerted through PRs and THP and their opposing role in regulating seizures, catamenial epilepsy, and seizure exacerbation associated with progesterone withdrawal.

Enhancement of Muscimol Binding and Gating by Allosteric Modulators of the GABAA Receptor: Relating Occupancy to State Functions

Muscimol is a psychoactive isoxazole derived from the mushroom Amanita muscaria and a potent orthosteric agonist of the GABAA receptor. The binding of [3H]muscimol has been used to evaluate the distribution of GABAA receptors in the brain, and studies of modulation of [3H]muscimol binding by allosteric GABAergic modulators such as barbiturates and steroid anesthetics have provided insight into the modes of action of these drugs on the GABAA receptor. It has, however, not been feasible to directly apply interaction parameters derived from functional studies to describe the binding of muscimol to the receptor. Here, we employed the Monod-Wyman-Changeux concerted transition model to analyze muscimol binding isotherms. We show that the binding isotherms from recombinant α1β3 GABAA receptors can be qualitatively predicted using electrophysiological data pertaining to properties of receptor activation and desensitization in the presence of muscimol. The model predicts enhancement of [3H]muscimol binding in the presence of the steroids allopregnanolone and pregnenolone sulfate, although the steroids interact with distinct sites and either enhance (allopregnanolone) or reduce (pregnenolone sulfate) receptor function. We infer that the concerted transition model can be used to link radioligand binding and electrophysiological data. SIGNIFICANCE STATEMENT: The study employs a three-state resting-active-desensitized model to link radioligand binding and electrophysiological data. We show that the binding isotherms can be qualitatively predicted using parameters estimated in electrophysiological experiments and that the model accurately predicts the enhancement of [3H]muscimol binding in the presence of the potentiating steroid allopregnanolone and the inhibitory steroid pregnenolone sulfate.

Mild chronic perturbation of inhibition severely alters hippocampal function

Pentameric GABAA receptors mediate a large share of CNS inhibition. The γ2 subunit is a typical constituent. At least 11 mutations in the γ2 subunit cause human epilepsies, making the role of γ2-containing receptors in brain function of keen basic and translational interest. How small changes to inhibition may cause brain abnormalities, including seizure disorders, is unclear. In mice, we perturbed fast inhibition with a point mutation T272Y (T6'Y in the second membrane-spanning domain) to the γ2 subunit. The mutation imparts resistance to the GABAA receptor antagonist picrotoxin, allowing verification of mutant subunit incorporation. We confirmed picrotoxin resistance and biophysical properties in recombinant receptors. T6'Y γ2-containing receptors also exhibited faster deactivation but unaltered steady-state properties. Adult T6'Y knockin mice exhibited myoclonic seizures and abnormal cortical EEG, including abnormal hippocampal-associated theta oscillations. In hippocampal slices, picrotoxin-insensitive inhibitory synaptic currents exhibited fast decay. Excitatory/inhibitory balance was elevated by an amount expected from the IPSC alteration. Partial pharmacological correction of γ2-mediated IPSCs with diazepam restored total EEG power toward baseline, but had little effect on the abnormal low-frequency peak in the EEG. The results suggest that at least part of the abnormality in brain function arises from the acute effects of truncated inhibition.

Steady-state activation of the high-affinity isoform of the α4β2δ GABAA receptor

Activation of GABA_A receptors consisting of α4, β2 (or β3), and δ subunits is a major contributor to tonic inhibition in several brain regions. The goal of this study was to analyze the function of the α4β2δ receptor in the presence of GABA and other endogenous and clinical activators and modulators under steady-state conditions. We show that the receptor has a high constitutive open probability (~0.1), but is only weakly activated by GABA that has a maximal peak open probability (P_Open,peak) of 0.4, taurine (maximal P_Open,peak = 0.4), or the endogenous steroid allopregnanolone (maximal P_Open,peak = 0.2). The intravenous anesthetic propofol is a full agonist (maximal P_Open,peak = 0.99). Analysis of currents using a cyclic three-state Resting-Active-Desensitized model indicates that the maximal steady-state open probability of the α4β2δ receptor is ~0.45. Steady-state open probability in the presence of combinations of GABA, taurine, propofol, allopregnanolone and/or the inhibitory steroid pregnenolone sulfate closely matched predicted open probability calculated assuming energetic additivity. The results suggest that the receptor is active in the presence of physiological concentrations of GABA and taurine, but, surprisingly, that receptor activity is only weakly potentiated by propofol.

Neurosteroids as novel antidepressants and anxiolytics: GABA-A receptors and beyond

The recent FDA approval of the neurosteroid, brexanolone (allopregnanolone), as a treatment for women with postpartum depression, and successful trials of a related neuroactive steroid, SGE-217, for men and women with major depressive disorder offer the hope of a new era in treating mood and anxiety disorders based on the potential of neurosteroids as modulators of brain function. This review considers potential mechanisms contributing to antidepressant and anxiolytic effects of allopregnanolone and other GABAergic neurosteroids focusing on their actions as positive allosteric modulators of GABAA receptors. We also consider their roles as endogenous "stress" modulators and possible additional mechanisms contributing to their therapeutic effects. We argue that further understanding of the molecular, cellular, network and psychiatric effects of neurosteroids offers the hope of further advances in the treatment of mood and anxiety disorders.

Steady-state activation and modulation of the synaptic-type α1β2γ2L GABAA receptor by combinations of physiological and clinical ligands

The synaptic α1β2γ2 GABAA receptor is activated phasically by presynaptically released GABA. The receptor is considered to be inactive between synaptic events when exposed to ambient GABA because of its low resting affinity to the transmitter. We tested the hypothesis that a combination of physiological and/or clinical positive allosteric modulators of the GABAA receptor with ambient GABA generates measurable steady-state activity. Recombinant α1β2γ2L GABAA receptors were expressed in Xenopus oocytes and activated by combinations of low concentrations of orthosteric (GABA, taurine) and allosteric (the steroid allopregnanolone, the anesthetic propofol) agonists, in the absence and presence of the inhibitory steroid pregnenolone sulfate. Steady-state activity was analyzed using the three-state cyclic Resting-Active-Desensitized model. We estimate that the steady-state open probability of the synaptic α1β2γ2L GABAA receptor in the presence of ambient GABA (1 μmol/L), taurine (10 μmol/L), and physiological levels of allopregnanolone (0.01 μmol/L) and pregnenolone sulfate (0.1 μmol/L) is 0.008. Coapplication of a clinical concentration of propofol (1 μmol/L) increases the steady-state open probability to 0.03. Comparison of total charge transfer for phasic and tonic activity indicates that steady-state activity can contribute strongly (~20 to >99%) to integrated activity from the synaptic GABAA receptor.

Steady-State Activation and Modulation of the Concatemeric α1β2γ2L GABAA Receptor

The two-state coagonist model has been successfully used to analyze and predict peak current responses of the γ-aminobutyric acid type A (GABA_A) receptor. The goal of the present study was to provide a model-based description of GABA_A receptor activity under steady-state conditions after desensitization has occurred. We describe the derivation and properties of the cyclic three-state resting-active-desensitized (RAD) model. The relationship of the model to receptor behavior was tested using concatemeric α1β2γ2 GABA_A receptors expressed in Xenopus oocytes. The receptors were activated by the orthosteric agonists GABA or β-alanine, the allosteric agonist propofol, or combinations of GABA, propofol, pentobarbital, and the steroid allopregnanolone, and the observed steady-state responses were compared with those predicted by the model. A modified RAD model was employed to analyze and describe the actions on steady-state current of the inhibitory steroid pregnenolone sulfate. The findings indicate that the steady-state activity in the presence of multiple active agents that interact with distinct binding sites follows standard energetic additivity. The derived equations enable prediction of peak and steady-state activity in the presence of orthosteric and allosteric agonists, and the inhibitory steroid pregnenolone sulfate. SIGNIFICANCE STATEMENT: The study describes derivation and properties of a three-state resting-active-desensitized model. The model and associated equations can be used to analyze and predict peak and steady-state activity in the presence of one or more active agents.

A Clickable Oxysterol Photolabel Retains NMDA Receptor Activity and Accumulates in Neurons

Oxysterol analogs that modulate NMDA receptor function are candidates for therapeutic development to treat neuropsychiatric disorders. However, the cellular actions of these compounds are still unclear. For instance, how these compounds are compartmentalized or trafficked in neurons is unknown. In this study, we utilized a chemical biology approach combining photolabeling and click chemistry. We introduce a biologically active oxysterol analog that contains: (1) a diazirine group, allowing for the permanent labeling of cellular targets, and (2) an alkyne group, allowing for subsequent in situ visualization using Cu²⁺ catalyzed cycloaddition of an azide-conjugated fluorophore. The physiological properties of this analog at NMDA receptors resemble those of other oxysterols, including occlusion with other oxysterol-like compounds. Fluorescent imaging reveals that the analog accumulates diffusely in the cytoplasm of neurons through an energy-independent mechanism. Overall, this work introduces a novel chemical biology approach to investigate oxysterol actions and introduces a tool useful for further cell biological and biochemical studies of oxysterols.

Visualizing pregnenolone sulfate-like modulators of NMDA receptor function reveals intracellular and plasma-membrane localization

Positive modulators of NMDA receptors are important candidates for therapeutic development to treat psychiatric disorders including autism and schizophrenia. Sulfated neurosteroids have been studied as positive allosteric modulators of NMDA receptors for years, but we understand little about the cellular fate of these compounds, an important consideration for drug development. Here we focus on a visualizable sulfated neurosteroid analogue, KK-169. As expected of a pregnenolone sulfate analogue, the compound strongly potentiates NMDA receptor function, is an antagonist of GABA_A receptors, exhibits occlusion with pregnenolone sulfate potentiation, and requires receptor domains important for pregnenolone sulfate potentiation. KK-169 exhibits somewhat higher potency than the natural parent, pregnenolone sulfate. The analogue contains a side-chain alkyne group, which we exploited for retrospective click labeling of neurons. Although the anionic sulfate group is expected to hinder cell entry, we detected significant accumulation of KK-169 in neurons with even brief incubations. Adding a photolabile diazirine group revealed that the expected plasma membrane localization of KK-169 is likely lost during fixation. Overall, our studies reveal new facets of the structure-activity relationship of neurosteroids at NMDA receptors, and their intracellular distribution suggests that sulfated neurosteroids could have unappreciated targets in addition to plasma membrane receptors.

Chemogenetic Isolation Reveals Synaptic Contribution of δ GABAA Receptors in Mouse Dentate Granule Neurons

Two major GABA_A receptor classes mediate ionotropic GABA signaling, those containing a δ subunit and those with a γ2 subunit. The classical viewpoint equates γ2-containing receptors with IPSCs and δ-containing receptors with tonic inhibition because of differences in receptor localization, but significant questions remain because the populations cannot be pharmacologically separated. We removed this barrier using gene editing to confer a point mutation on the δ subunit in mice, rendering receptors containing the subunit picrotoxin resistant. By pharmacologically isolating δ-containing receptors, our results demonstrate their contribution to IPSCs in dentate granule neurons and weaker contributions to thalamocortical IPSCs. Despite documented extrasynaptic localization, we found that receptor localization does not preclude participation in isolated IPSCs, including mIPSCs. Further, phasic inhibition from δ subunit-containing receptors strongly inhibited summation of EPSPs, whereas tonic activity had little impact. In addition to any role that δ-containing receptors may play in canonical tonic inhibition, our results highlight a previously underestimated contribution of δ-containing receptors to phasic inhibition.SIGNIFICANCE STATEMENT GABA_A receptors play key roles in transient and tonic inhibition. The prevailing view suggests that synaptic γ2-containing GABA_A receptors drive phasic inhibition, whereas extrasynaptic δ-containing receptors mediate tonic inhibition. To re-evaluate the impact of δ receptors, we took a chemogenetic approach that offers a sensitive method to probe the synaptic contribution of δ-containing receptors. Our results reveal that localization does not strongly limit the contribution of δ receptors to IPSCs and that δ receptors make an unanticipated robust contribution to phasic inhibition.

The dual-gate model for pentameric ligand-gated ion channels activation and desensitization

Pentameric ligand-gated ion channels (pLGICs) mediate fast neurotransmission in the nervous system. Their dysfunction is associated with psychiatric, neurological and neurodegenerative disorders such as schizophrenia, epilepsy and Alzheimer's disease. Understanding their biophysical and pharmacological properties, at both the functional and the structural level, thus holds many therapeutic promises. In addition to their agonist-elicited activation, most pLGICs display another key allosteric property, namely desensitization, in which they enter a shut state refractory to activation upon sustained agonist binding. While the activation mechanisms of several pLGICs have been revealed at near-atomic resolution, the structural foundation of desensitization has long remained elusive. Recent structural and functional data now suggest that the activation and desensitization gates are distinct, and are located at both sides of the ion channel. Such a 'dual gate mechanism' accounts for the marked allosteric effects of channel blockers, a feature illustrated herein by theoretical kinetics simulations. Comparison with other classes of ligand- and voltage-gated ion channels shows that this dual gate mechanism emerges as a common theme for the desensitization and inactivation properties of structurally unrelated ion channels.

Probing GABAA receptors with inhibitory neurosteroids

γ-aminobutyric acid type A receptors (GABA_ARs) are important components of the central nervous system and they are functionally tasked with controlling neuronal excitability. These receptors are subject to post-translational modification and also to modulation by endogenous regulators, such as the neurosteroids. These modulators can either potentiate or inhibit GABA_AR function. Whilst the former class of neurosteroids are considered to bind to and act from the transmembrane domain of the receptor, the domains that are important for the inhibitory neurosteroids remain less clear. In this study, we systematically compare a panel of recombinant synaptic-type and extrasynaptic-type GABA_ARs expressed in heterologous cell systems for their sensitivity to inhibition by the classic inhibitory neurosteroid, pregnenolone sulphate. Generally, peak GABA current responses were inhibited less compared to steady-state currents, implicating the desensitised state in inhibition. Moreover, pregnenolone sulphate inhibition increased with GABA concentration, but showed minimal voltage dependence. There was no strong dependence of inhibition on receptor subunit composition, the exception being the ρ1 receptor, which is markedly less sensitive. By using competition experiments with pregnenolone sulphate and the GABA channel blocker picrotoxinin, discrete binding sites are proposed. Furthermore, by assessing inhibition using site-directed mutagenesis and receptor chimeras comprising α, β or γ subunits with ρ1 subunits, the receptor transmembrane domains are strongly implicated in mediating inhibition and most likely the binding location for pregnenolone sulphate in GABA_ARs.

This article is part of the “Special Issue Dedicated to Norman G. Bowery”.

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A range of GABA_A receptor subtypes are inhibited by pregenolone sulphate.

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Peak GABA curents are less sensitive to inhibition than steady-state currents.

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Desensitised state of GABA_A receptors most sensitive to neurosteroid inhibition.

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Inhibition increases with GABA concentration, but not strongly voltage-dependent.

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Pregnenolone sulphate binding site located within subunit transmembrane domains.

Neurosteroid biosynthesis down-regulation and changes in GABAA receptor subunit composition: a biomarker axis in stress-induced cognitive and emotional impairment

By rapidly modulating neuronal excitability, neurosteroids regulate physiological processes, such as responses to stress and development. Excessive stress affects their biosynthesis and causes an imbalance in cognition and emotions. The progesterone derivative, allopregnanolone (Allo) enhances extrasynaptic and postsynaptic inhibition by directly binding at GABA_A receptors, and thus, positively and allosterically modulates the function of GABA. Allo levels are decreased in stress-induced psychiatric disorders, including depression and post-traumatic stress disorder (PTSD), and elevating Allo levels may be a valid therapeutic approach to counteract behavioural dysfunction. While benzodiazepines are inefficient, selective serotonin reuptake inhibitors (SSRIs) represent the first choice treatment for depression and PTSD. Their mechanisms to improve behaviour in preclinical studies include neurosteroidogenic effects at low non-serotonergic doses. Unfortunately, half of PTSD and depressed patients are resistant to current prescribed 'high' dosage of these drugs that engage serotonergic mechanisms. Unveiling novel biomarkers to develop more efficient treatment strategies is in high demand. Stress-induced down-regulation of neurosteroid biosynthesis and changes in GABA_A receptor subunit expression offer a putative biomarker axis to develop new PTSD treatments. The advantage of stimulating Allo biosynthesis relies on the variety of neurosteroidogenic receptors to be targeted, including TSPO and endocannabinoid receptors. Furthermore, stress favours a GABA_A receptor subunit composition with higher sensitivity for Allo. The use of synthetic analogues of Allo is a valuable alternative. Pregnenolone or drugs that stimulate its levels increase Allo but also sulphated steroids, including pregnanolone sulphate which, by inhibiting NMDA tonic neurotransmission, provides neuroprotection and cognitive benefits. In this review, we describe current knowledge on the effects of stress on neurosteroid biosynthesis and GABA_A receptor neurotransmission and summarize available pharmacological strategies that by enhancing neurosteroidogenesis are relevant for the treatment of SSRI-resistant patients. Linked Articles This article is part of a themed section on Pharmacology of Cognition: a Panacea for Neuropsychiatric Disease? To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v174.19/issuetoc.

Interaction between positive allosteric modulators and trapping blockers of the NMDA receptor channel

BACKGROUND AND PURPOSE

Memantine and ketamine are clinically used, open-channel blockers of NMDA receptors exhibiting remarkable pharmacodynamic similarities despite strikingly different clinical profiles. Although NMDA channel gating constitutes an important difference between memantine and ketamine, it is unclear how positive allosteric modulators (PAMs) might affect the pharmacodynamics of these NMDA blockers.

EXPERIMENTAL APPROACH

We used two different PAMs: SGE-201, an analogue of an endogenous oxysterol, 24S-hydroxycholesterol, along with pregnenolone sulphate (PS), to test on memantine and ketamine responses in single cells (oocytes and cultured neurons) and networks (hippocampal slices), using standard electrophysiological techniques.

KEY RESULTS

SGE-201 and PS had no effect on steady-state block or voltage dependence of a channel blocker. However, both PAMs increased the actions of memantine and ketamine on phasic excitatory post-synaptic currents, but neither revealed underlying pharmacodynamic differences. SGE-201 accelerated the re-equilibration of blockers during voltage jumps. SGE-201 also unmasked differences among the blockers in neuronal networks - measured either by suppression of activity in multi-electrode arrays or by neuroprotection against a mild excitotoxic insult. Either potentiating NMDA receptors while maintaining the basal activity level or increasing activity/depolarization without potentiating NMDA receptor function is sufficient to expose pharmacodynamic blocker differences in suppressing network function and in neuroprotection.

CONCLUSIONS AND IMPLICATIONS

Positive modulation revealed no pharmacodynamic differences between NMDA receptor blockers at a constant voltage, but did expose differences during spontaneous network activity. Endogenous modulator tone of NMDA receptors in different brain regions may underlie differences in the effects of NMDA receptor blockers on behaviour.

Phosphatidylinositol 4,5-bisphosphate depletion fails to affect neurosteroid modulation of GABAA receptor function

RATIONALE

Neurosteroids and likely other lipid modulators access transmembrane sites on the GABAA receptor (GABAAR) by partitioning into and diffusing through the plasma membrane. Therefore, specific components of the plasma membrane may affect the potency or efficacy of neurosteroid-like modulators. Here, we tested a possible role for phosphatidylinositol 4,5-bisphosphate (PIP2), a phospholipid that governs activity of many channels and transporters, in modulation or function of GABAARs.

OBJECTIVES

In these studies, we sought to deplete plasma-membrane PIP2 and probe for a change in the strength of potentiation by submaximal concentrations of the neurosteroid allopregnanolone (3α5αP) and other anesthetics, including propofol, pentobarbital, and ethanol. We also tested for a change in the behavior of negative allosteric modulators pregnenolone sulfate and dipicrylamine.

METHODS

We used Xenopus oocytes expressing the ascidian voltage-sensitive phosphatase (Ci-VSP) to deplete PIP2. Voltage pulses to positive membrane potentials were used to deplete PIP2 in Ci-VSP-expressing cells. GABAARs composed of α1β2γ2L and α4β2δ subunits were challenged with GABA and 3α5αP or other modulators before and after PIP2 depletion. KV7.1 channels and NMDA receptors (NMDARs) were used as positive controls to verify PIP2 depletion.

RESULTS

We found no evidence that PIP2 depletion affected modulation of GABAARs by positive or negative allosteric modulators. By contrast, Ci-VSP-induced PIP2 depletion depressed KV7.1 activation and NMDAR activity.

CONCLUSIONS

We conclude that despite a role for PIP2 in modulation of a wide variety of ion channels, PIP2 does not affect modulation of GABAARs by neurosteroids or related compounds.

Neurosteroid and neurotransmitter alterations in Parkinson's disease

Parkinson's disease (PD) is associated with a massive loss of dopaminergic cells in the substantia nigra leading to dopamine hypofunction and alteration of the basal ganglia circuitry. These neurons, are under the control, among others, of the excitatory glutamatergic and inhibitory γ-aminobutyric acid (GABA) systems. An imbalance between these systems may contribute to excitotoxicity and dopaminergic cell death. Neurosteroids, a group of steroid hormones synthesized in the brain, modulate the function of several neurotransmitter systems. The substantia nigra of the human brain expresses high concentrations of allopregnanolone (3α, 5αtetrahydroprogesterone), a neurosteroid that positively modulates the action of GABA at GABAA receptors and of 5α-dihydroprogesterone, a neurosteroid acting at the genomic level. This article reviews the roles of NS acting as neuroprotectants and as GABAA receptor agonists in the physiology and pathophysiology of the basal ganglia, their impact on dopaminergic cell activity and survival, and potential therapeutic application in PD.

Neurosteroids, stress and depression: potential therapeutic opportunities

Neurosteroids are potent and effective neuromodulators that are synthesized from cholesterol in the brain. These agents and their synthetic derivatives influence the function of multiple signaling pathways including receptors for γ-aminobutyric acid (GABA) and glutamate, the major inhibitory and excitatory neurotransmitters in the central nervous system (CNS). Increasing evidence indicates that dysregulation of neurosteroid production plays a role in the pathophysiology of stress and stress-related psychiatric disorders, including mood and anxiety disorders. In this paper, we review the mechanisms of neurosteroid action in brain with an emphasis on those neurosteroids that potently modulate the function of GABA(A) receptors. We then discuss evidence indicating a role for GABA and neurosteroids in stress and depression, and focus on potential strategies that can be used to manipulate CNS neurosteroid synthesis and function for therapeutic purposes.

Characteristics of concatemeric GABA(A) receptors containing α4/δ subunits expressed in Xenopus oocytes

BACKGROUND AND PURPOSE

GABA(A) receptors mediate both synaptic and extrasynaptic actions of GABA. In several neuronal populations, α4 and δ subunits are key components of extrasynaptic GABA(A) receptors that strongly influence neuronal excitability and could mediate the effects of neuroactive agents including neurosteroids and ethanol. However, these receptors can be difficult to study in native cells and recombinant δ subunits can be difficult to express in heterologous systems.

EXPERIMENTAL APPROACH

We engineered concatemeric (fused) subunits to ensure δ and α4 subunit expression. We tested the pharmacology of the concatemeric receptors, compared with a common synaptic-like receptor subunit combination (α1 +β2 +γ2L), and with free-subunit α4/δ receptors, expressed in Xenopus oocytes.

KEY RESULTS

δ-β2 -α4 +β2-α4 cRNA co-injected into Xenopus oocytes resulted in GABA-gated currents with the expected pharmacological properties of α4/δ-containing receptors. Criteria included sensitivity to agonists of different efficacy, sensitivity to the allosteric activator pentobarbital, and modulation of agonist responses by DS2 (4-chloro-N-[2-(2-thienyl)imidazo[1,2-a]pyridine-3-yl benzamide; a δ-selective positive modulator), furosemide, and Zn(2+) . We used the concatemers to examine neurosteroid sensitivity of extrasynaptic-like, δ-containing receptors. We found no qualitative differences between extrasynaptic-like receptors and synaptic-like receptors in the actions of either negative or positive neurosteroid modulators of receptor function. Quantitative differences were explained by the partial agonist effects of the natural agonist GABA and by a mildly increased sensitivity to low steroid concentrations.

CONCLUSIONS AND IMPLICATIONS

The neurosteroid structure-activity profile for α4/δ-containing extrasynaptic receptors is unlikely to differ from that of synaptic-like receptors such as α1/β2/γ2-containing receptors.

Hydrophobic anions potently and uncompetitively antagonize GABA(A) receptor function in the absence of a conventional binding site

BACKGROUND AND PURPOSE

A 'lock-and-key' binding site typically accounts for the effect of receptor antagonists. However, sulphated neurosteroids are potent non-competitive antagonists of GABA(A) receptors without a clear structure-activity relationship. To gain new insights, we tested two structurally unrelated hydrophobic anions with superficially similar properties to sulphated neurosteroids.

EXPERIMENTAL APPROACH

We used voltage-clamp techniques in Xenopus oocytes and hippocampal neurons to characterize dipicrylamine (DPA) and tetraphenylborate (TPB), compounds previously used to probe membrane structure and voltage-gated ion channel function.

KEY RESULTS

Both DPA and TPB potently antagonized GABA(A) receptors. DPA exhibited an IC₅₀ near 60 nM at half-maximal GABA concentration and antagonism with features indistinguishable from pregnenolone sulphate antagonism, including sensitivity to a point mutation in transmembrane domain 2 of the α1 subunit. Bovine serum albumin, which scavenges free membrane-associated DPA, accelerated both capacitance offset and antagonism washout. Membrane interactions and antagonism were explored using the voltage-dependent movement of DPA between membrane leaflets. Washout of DPA antagonism was strongly voltage-dependent, paralleling DPA membrane loss, although steady-state antagonism lacked voltage dependence. At antagonist concentrations, DPA failed to affect inhibitory post-synaptic current (IPSC) amplitude or decay, but DPA accelerated pharmacologically prolonged IPSCs.

CONCLUSIONS AND IMPLICATIONS

Neurosteroid-like GABA(A) receptor antagonism appears to lacks a conventional binding site. These features highlight key roles of membrane interactions in antagonism. Because its membrane mobility can be controlled, DPA may be a useful probe of GABA(A) receptors, but its effects on excitability via GABA(A) receptors raise caveats for its use in monitoring neuronal activity.

Continuous de novo synthesis of neurosteroids is required for normal synaptic transmission and plasticity in the dentate gyrus of the rat hippocampus

Both in vivo and in vitro studies have shown that neurosteroids promote learning and memory by modulating synaptic functions in the hippocampus. However, we do not know to what degree endogenously synthesized neurosteroids contribute to the hippocampal synaptic functions. Cytochrome P450scc is the enzyme that converts cholesterol to pregnenolone (PREG), which is required for the biosynthesis of all other neurosteroids. To investigate the physiological roles of endogenous neurosteroids in synaptic functions, we electrophysiologically examined the effects of aminoglutethimide (AG), a selective inhibitor of P450scc, on the synaptic transmission and plasticity in the dentate gyrus of rat hippocampal slices. The application of AG (100 μM) decreased the slope of the field excitatory postsynaptic potentials (fEPSPs) in granule cells by 20-30% in 20 min through the modulation of postsynaptic AMPA receptors, while it did not affect the presynaptic properties, including the paired-pulse ratio and the probability of glutamate release from presynaptic terminals. The AG-induced depression was nearly completely rescued by exogenously applied 500 nM PREG or by 1 nM dehydroepiandrosterone sulfate (DHEAS), one of the neurosteroids synthesized from PREG, suggesting that the AG-induced depression was caused by the loss of DHEAS. AG also reduced NMDA receptor activity, and suppressed high-frequency stimulation (HFS)-induced long-term potentiation (LTP). These findings provide novel evidence that the endogenous neurosteroids locally synthesized in the brain are required to maintain the normal excitatory synaptic transmission and plasticity in the dentate gyrus of the rat hippocampus.

Is connexin36 critical for GABAergic hypersynchronization in the hippocampus?

Synchronous bursting of cortical GABAergic interneurons is important in epilepsies associated with excitatory GABAergic signalling. If electrical coupling was critical for the generation of this pathological activity, then the development of selective blockers of connexin36-based interneuronal gap junctions could be of therapeutic value. We have addressed this issue in the 4-aminopyridine model of epilepsy in vitro by comparing GABAergic epileptiform currents and their sensitivity to gap junction blockers in wild-type vs. connexin36 knockout mice. Although electrical coupling was abolished in stratum lacunosum-moleculare interneurons from knockout animals, epileptiform currents were not eliminated. Furthermore, epileptiform currents propagated similarly across hippocampal layers in the two genotypic groups. Blockade of electrical coupling with carbenoxolone suppressed amplitude, frequency and half-width of the epileptiform currents both in wild-type and in knockout animals, whereas mefloquine had no effects. Carbenoxolone also depressed responses to exogenous and synaptic GABA application onto interneurons. We conclude that, in the 4-aminopyridine model of epilepsy in vitro, connexin36 is not critical for the generation of epileptiform discharges in GABAergic networks and that the observed antiepileptic effects of carbenoxolone are likely to be due to blockade of GABAA receptors and not of connexin36-based gap junctions. Lastly, because of its chemical structure and its effects on amplitude and kinetics of GABAergic currents, we tested the hypothesis that carbenoxolone acted via specific sites on GABAA receptors, such as the one mediating the effects of the neurosteroid pregnenolone sulfate, or the allosteric regulatory site of benzodiazepines/β-carbolines. Our results suggest that neither of these is involved.

Neurosteroids and GABA-A Receptor Function

Neurosteroids represent a class of endogenous steroids that are synthesized in the brain, the adrenals, and the gonads and have potent and selective effects on the GABAA-receptor. 3α-hydroxy A-ring reduced metabolites of progesterone, deoxycorticosterone, and testosterone are positive modulators of GABA(A)-receptor in a non-genomic manner. Allopregnanolone (3α-OH-5α-pregnan-20-one), 5α-androstane-3α, 17α-diol (Adiol), and 3α5α-tetrahydrodeoxycorticosterone (3α5α-THDOC) enhance the GABA-mediated Cl(-) currents acting on a site (or sites) distinct from the GABA, benzodiazepine, barbiturate, and picrotoxin binding sites. 3α5α-P and 3α5α-THDOC potentiate synaptic GABA(A)-receptor function and activate δ-subunit containing extrasynaptic receptors that mediate tonic currents. On the contrary, 3β-OH pregnane steroids and pregnenolone sulfate (PS) are GABA(A)-receptor antagonists and induce activation-dependent inhibition of the receptor. The activities of neurosteroid are dependent on brain regions and types of neurons. In addition to the slow genomic action of the parent steroids, the non-genomic, and rapid actions of neurosteroids play a significant role in the GABA(A)-receptor function and shift in mood and memory function. This review describes molecular mechanisms underlying neurosteroid action on the GABA(A)-receptor, mood changes, and cognitive functions.

The neurosteroid dehydroepiandrosterone (DHEA) and its metabolites alter 5-HT neuronal activity via modulation of GABAA receptors

Dehydroepiandrosterone (DHEA) and its metabolites, DHEA-sulphate (DHEA-S) and androsterone, have neurosteroid activity. In this study, we examined whether DHEA, DHEA-S and androsterone, can influence serotonin (5-HT) neuronal firing activity via modulation of γ-aminobutryic acid (GABA(A)) receptors. The firing of presumed 5-HT neurones in a slice preparation containing rat dorsal raphe nucleus was inhibited by the GABA(A) receptor agonists 4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridinyl-3-ol (THIP) (25 μM) and GABA (100 μM). DHEA (100 and 300 μM) and DHEA-S (1, 10 and 100 μM) caused a rapid and reversible attenuation of the response to THIP. DHEA (100 μM) and DHEA-S (100 μM) also attenuated the effect of GABA. Androsterone (10 and 30 μM) markedly enhanced the inhibitory response to THIP (25 μM). The effect was apparent during androsterone administration but persisted and even increased in magnitude after drug wash-out. The data indicate that GABA(A) receptor-mediated regulation of 5-HT neuronal firing is sensitive to negative modulation by DHEA and its metabolite DHEA-S is sensitive to positive modulation by the metabolite androsterone. The effects of these neurosteroids on GABA(A) receptor-mediated regulation of 5-HT firing may underlie some of the reported behavioural and psychological effects of endogenous and exogenous DHEA.

Structurally diverse amphiphiles exhibit biphasic modulation of GABAA receptors: similarities and differences with neurosteroid actions

BACKGROUND AND PURPOSE

Some neurosteroids, notably 3alpha-hydroxysteroids, positively modulate GABA(A) receptors, but sulphated steroids negatively modulate these receptors. Recently, other lipophilic amphiphiles have been suggested to positively modulate GABA receptors. We examined whether there was similarity among the actions of these agents and the mechanisms of neurosteroids. Significant similarity would affect theories about the specificity of steroid actions.

EXPERIMENTAL APPROACH

Xenopus laevis oocytes were challenged with Triton X-100, octyl-beta-glucoside, capsaicin, docosahexaenoic acid and sodium dodecyl sulphate (SDS), along with different GABA concentrations.

KEY RESULTS

These compounds have both positive and negative effects on GABA currents, which can be accentuated according to the degree of receptor activation. A low GABA concentration (1 microM) promoted potentiation and a high concentration (20 microM) promoted inhibition of current, except for SDS that inhibited function even at low GABA concentrations. Amphiphile inhibition was characterized by enhanced apparent desensitization and by weak voltage dependence, similar to pregnenolone sulphate antagonism. We then tested amphiphile effects on mutated receptor subunits that are insensitive to negative (alpha1V256S) and positive (alpha1Q241L or alpha1N407A/Y410F) steroid modulation. Negative regulation by amphiphiles was nearly abolished in alpha1V256S-mutated receptors, but potentiation was unaffected. In alpha1Q241L- or alpha1N407A/Y410F-mutated receptors, potentiation by amphiphiles remained intact.

CONCLUSIONS AND IMPLICATIONS

Structurally diverse amphiphiles have antagonist actions at GABA(A) receptors very similar to those of sulphated neurosteroids, while the potentiating mechanisms of these amphiphiles are distinct from those of neurosteroid-positive modulators. Thus, such antagonism at GABA(A) receptors does not have a clear pharmacophore requirement.

The influence of the membrane on neurosteroid actions at GABA(A) receptors

Modern views of anesthetic neurosteroid interaction with the GABA(A) receptor conceptualize steroid ligands interacting with a protein binding site on the receptor. It has generally been assumed that the steroid interaction/binding site is contained in an extracellular domain of the receptor, and that steroid interactions are of high potency, evidenced by the low aqueous ligand concentrations required to achieve potentiation of channel function. We have been considering implications of the observations that steroids are quite lipophilic and that recently identified putative steroid binding sites are in transmembrane domains of the receptor. Accordingly, we expect that both the effective plasma membrane steroid concentration and steroid pharmacophore properties will contribute to steady-state potency and to the lifetime of steroid actions following removal of free aqueous steroid. Here we review our recent studies that address the evidence that membrane partitioning and intracellular accumulation are non-specific contributors to the effects of anesthetic steroids at GABA(A) receptors. We compare and contrast the profile of anesthetic steroids with that of sulfated steroids that negatively regulate GABA(A) receptor function. These studies give rise to the view that the inherent affinity of anesthetic steroid for GABA(A) receptors is very low; low effective aqueous concentrations are accounted for by lipid partitioning. This yields a very different picture of the interaction of neurosteroids with the GABA(A) receptor than that of steroid interactions with classical intracellular steroid receptors, which exhibit inherently high affinity. These considerations have practical implications for actions of endogenous neurosteroids. Lipophilicity will tend to promote autocrine actions of neurosteroids at GABA(A) receptors within cells that synthesize neurosteroids, and lipophilic retention will limit intercellular diffusion from the source of steroid synthesis. Lipophilicity and steroid access to the receptor binding sites also must be considerations in drug design if drugs are to effectively reach the target GABA(A) receptor site.

NMDA potentiation by visible light in the presence of a fluorescent neurosteroid analogue

N-Methyl-D-aspartate (NMDA) receptors are widely studied because of their importance in synaptic plasticity and excitotoxic cell death. Here we report a novel method of potentiating NMDA receptors with fluorescence excited by blue (480 nm) light. In the presence of 300 nM of a (7-nitro-2,1,3-benzoxadiazol-4-yl) amino (NBD)-tagged neuroactive steroid carrier C2-NBD-(3alpha,5alpha)-3-hydroxypregnan-20-one (C2-NBD 3alpha5alphaP), responses of cultured hippocampal neurons to 10 microM NMDA were potentiated to 219.2 +/- 9.2% of the baseline response (100%) by a 30 s exposure to 480 nm light. The potentiation decayed back to baseline with a time constant of 80.6 s. Responses to 1 microM and 100 microM NMDA were potentiated to 147.9 +/- 9.6% and 174.1 +/- 15.6% of baseline, respectively, suggesting that visible-light potentiation is relatively insensitive to NMDA concentration. Peak autaptic NMDA responses were potentiated to 178.9 +/- 22.4% of baseline. Similar potentiation was seen with 10 microM NBD-lysine, suggesting that visible-light potentiation is not a steroid effect. Potentiation was also seen with a steroid analogue in which the NBD was replaced with fluorescein, suggesting that NBD is not the only fluorophore capable of supporting visible-light potentiation. UV light and redox potentiation of NMDA receptors largely occluded subsequent blue light potentiation (127.7 +/- 7.4% and 120.2 +/- 6.2% of baseline, respectively). The NR1a(C744A,C798A) mutant that is insensitive to redox and UV potentiation was also largely unaffected by visible-light potentiation (135.0 +/- 10.0% of baseline). Finally, we found that the singlet oxygen scavenger furfuryl alcohol decreased visible-light potentiation. Collectively, these data suggest that visible-light potentiation of NMDA receptors by fluorescence excitation shares mechanisms with UV and redox potentiation and may involve singlet oxygen production.

The effect of the neuroactive steroid 5beta-pregnane-3beta, 20(R)-diol on the time course of GABA evoked currents is different to that of pregnenolone sulphate

The endogenous progesterone metabolite allopregnanolone has a number of properties including anesthetic, sedative, antiepileptic, anxiolytic, impaired memory function and negative mood symptoms. Allopregnanolone is a potent positive GABA(A) receptor function modulators. In contrast, 3beta-hydroxy-steroids (3beta-steroids) usually modulate the GABA(A) receptor negatively. They have attracted some interest for their possible use as therapeutic agents that could counteract the negative symptoms induced by allopregnanolone. Two hypotheses for the action of 3beta-steroids have been proposed: 1) 3beta-steroids act in a similar way to pregnenolone sulphate, which non-competitively reduces GABA(A) receptor activity. 2) 3beta-steroids specifically antagonize the effect of allopregnanolone. We have therefore tried to clarify this issue by comparing the effect of pregnenolone sulphate and 5beta-pregnane-3beta, 20(R)-diol on the GABA-evoked currents by the patch clamp technique on neurons from the medial preoptic nucleus. Both pregnenolone sulphate and 5beta-pregnane-3beta, 20(R)-diol increase the desensitization rate of the current response evoked by a 2 s GABA application. However, their effects on other parameters of the GABA evoked currents differed in degree and sometimes even in direction. The actions of pregnenolone sulphate and 5beta-pregnane-3beta, 20(R)-diol were not altered in the presence of allopregnanolone, which indicates that they do not directly interact with allopregnanolone. In addition, when 5beta-pregnane-3beta, 20(R)-diol was tested on spontaneous inhibitory postsynaptic currents (sIPSCs), it dramatically reduced the allopregnanolone-induced prolongation of the decay time constant but it had no effect on the decay under control conditions. In conclusion, the effect of 5beta-pregnane-3beta, 20(R)-diol on GABA-evoked currents is different to that of pregnenolone sulphate in medial preoptic nucleus neurons.

Kinetic analysis of voltage-dependent potentiation and block of the glycine alpha 3 receptor by a neuroactive steroid analogue

We examined the actions of a carboxylated analogue of pregnanolone ((3alpha,5beta)-20-oxopregnane-3-carboxylic acid; 3alphaCOOH5betaP) on receptors composed of glycine receptor alpha3 subunits, expressed in Xenopus oocytes. This analogue both inhibits and potentiates this receptor; potentiation increases with more negative membrane potentials while block increases with less negative membrane potentials. We used a second analogue ((3alpha,5beta)-3-hydroxymethylpregnan-20-one; 3alphaCH(2)OH5betaP) to examine the mechanism for voltage-dependent potentiation. This analogue potentiates but does not block the glycine alpha3 receptor. Steady-state responses and current relaxations following voltage jumps support the idea that the voltage dependence of potentiation indirectly arises from a voltage dependence for channel activation by glycine, rather than an intrinsic voltage dependence for potentiation. Potentiation results from a slowing of the channel deactivation rate. In the absence of steroid, at a low [glycine] current relaxations after a voltage jump show two exponential components, with a weighted average time constant of approximately 425 ms (-50 mV, 22 degrees C). The rate for channel deactivation increases at more negative potentials (e-fold per 170 mV) whereas activation decreases (e-fold per 230 mV). The probability a channel is active at a high [glycine] is greater than 0.9. The addition of 10 microM 3alphaCH(2)OH5betaP decreases the deactivation rate by 6.3-fold (-50 mV). Voltage-dependent block by 3alphaCOOH5betaP is consistent with simple open-channel block, with voltage dependence reflecting interactions of the charge on the analogue with the electrical field. Block and unblock have equal but opposite dependence on membrane potential, and the charge on 3alphaCOOH5betaP senses approximately 70% of the membrane field at the blocking site. The apparent forward rate for block, however, is very slow (2 x 10(5) m(-1) s(-1)).

A TRP channel-steroid marriage

A surprising functional association between TRPM3, a mysterious member of the family of transient receptor potential (TRP) cation channels, and the sulphated version of pregnenolone, 'mother' of all steroid hormones, has been identified.

Effects on membrane capacitance of steroids with antagonist properties at GABAA receptors

We investigated the electrophysiological signature of neuroactive steroid interactions with the plasma membrane. We found that charged, sulfated neuroactive steroids, those that exhibit noncompetitive antagonism of GABA(A) receptors, altered capacitive charge movement in response to voltage pulses in cells lacking GABA receptors. Uncharged steroids, some of which are potent enhancers of GABA(A) receptor activity, produced no alteration in membrane capacitance. We hypothesized that the charge movements might result from physical translocation of the charged steroid through the transmembrane voltage, as has been observed previously with several hydrophobic anions. However, the charge movements and relaxation time constants of capacitive currents did not exhibit the Boltzmann-type voltage dependence predicted by a single barrier model. Further, a fluorescently tagged analog of a sulfated neurosteroid altered membrane capacitance similar to the parent compound but produced no voltage-dependent fluorescence change, a result inconsistent with a strong change in the polar environment of the fluorophore during depolarization. These findings suggest that negatively charged sulfated steroids alter the plasma membrane capacitance without physical movement of the molecule through the electric field.

Neurosteroids 3beta, 20 (R/S)-pregnandiols decrease offset rate of the GABA-site activation at the recombinant GABA A receptor

Neurosteroids directly modulate ligand gated ion channels such as GABA A receptors. Two such molecules, 3beta-OH A-ring reduced pregnane steroids and pregnenolone sulfate (PS), inhibit recombinant GABA A receptor. Using a two-electrode voltage-clamp technique, we compared the effect of 5alpha-pregnan-3beta,20(S)-diol (UC1019), 5beta-pregnan-3beta, 20(R)-diol (UC1020) and PS on the activation onset and offset times of the recombinant GABA A receptor (rat alpha1beta2gamma2L) in Xenopus oocytes. Rapid solution changes allowed the kinetic analysis of GABA-evoked currents. Steroids were co-applied with 30 microM GABA for 10 s, followed by a 80 s washout period. PS (> ir =0.3 microM) moderately increased the slow onset rate (k(on-S)) of GABA-response. PS had no significant effects on the fast onset rate (k(on-F)). UC1019 and UC1020 decreased the k(on-S) of the GABA-response in a concentration-dependent manner with no significant effects on the k(on-F). Like PS, UC1019 and UC1020 decreased the slow offset rates (k(off-S)). In addition, PS increased the fast offset rate (k(off-F)) in a concentration-dependent manner, while UC1019 and UC1020 decreased k(off-F). The EC50 of PS to increase k(off-F) was calculated as 0.47+/-0.1 microM. The corresponding IC50 values of UC1019 and UC1020 to decrease k(off-F) were 5.0+/-0.5 microM and 8.4+/-0.9 microM, respectively. These results suggest differential actions of PS and 3beta, 20(R/S)-pregnandiols on the offset time course of GABA-site activation.

Neuroactive steroids and human recombinant rho1 GABAC receptors

The gamma-aminobutyric acid type C (GABAC) receptor is structurally related to the GABAA receptors, yet quite distinct physiologically and pharmacologically. Neuroactive steroids are known to be potent and efficacious modulators of the GABAA receptor, but they are less well characterized in their actions on the GABAC receptor. We have examined the actions of neuroactive steroids and analogs on rho1 (GABAC) receptors expressed in Xenopus laevis oocytes, with two goals in mind. First, we tested a larger number of endogenous steroids, to determine whether particularly potent steroids could be found. Second, we examined the structure-activity relationship for steroid actions, and some mechanistic features, to determine the possible numbers of steroid binding sites and mechanisms of action. In total, 41 compounds were examined. Estradiols are inhibitors, essentially equipotent with picrotoxinin. No endogenous steroid tested was highly efficacious at potentiation. The results of the structure-activity studies and the effects of two mutations to the second transmembrane region of the rho1 GABAC receptor indicate that there are several mechanisms by which steroids can inhibit the receptor. Surprisingly, estradiol shares some features with picrotoxin. Inhibition by negatively charged compounds was not sensitive to membrane potential, and inhibition by all compounds tested was reduced at higher concentrations of GABA. The data indicate that the binding sites mediating potentiation and inhibition differ from each other and that there are several (three or more) mechanisms for producing inhibition.

Neurosteroid binding sites on GABA(A) receptors

Controlling neuronal excitability is vitally important for maintaining a healthy central nervous system (CNS) and this relies on the activity of type A gamma-aminobutyric acid (GABA(A)) neurotransmitter receptors. Given this role, it is therefore important to understand how these receptors are regulated by endogenous modulators in the brain and determine where they bind to the receptor. One of the most potent groups of modulators is the neurosteroids which regulate the activity of synaptic and extrasynaptic GABA(A) receptors. This level of regulation is thought to be physiologically important and its dysfunction may be relevant to numerous neurological conditions. The aim of this review is to summarise those studies that over the last 20 years have focussed upon finding the binding sites for neurosteroids on GABA(A) receptors. We consider the nature of steroid binding sites in other proteins where this has been determined at atomic resolution and how their generic features were mapped onto GABA(A) receptors to help locate 2 putative steroid binding sites. Altogether, the findings strongly suggest that neurosteroids do bind to discrete sites on the GABA(A) receptor and that these are located within the transmembrane domains of alpha and beta receptor subunits. The implications for neurosteroid binding to other inhibitory receptors such as glycine and GABA(C) receptors are also considered. Identifying neurosteroid binding sites may enable the precise pathophysiological role(s) of neurosteroids in the CNS to be established for the first time, as well as providing opportunities for the design of novel drug entities.

Mechanisms of neurosteroid interactions with GABA(A) receptors

Neuroactive steroids have some of their most potent actions by augmenting the function of GABA(A) receptors. Endogenous steroid actions on GABA(A) receptors may underlie important effects on mood and behavior. Exogenous neuroactive steroids have potential as anesthetics, anticonvulsants, and neuroprotectants. We have taken multiple approaches to understand more completely the interaction of neuroactive steroids with GABA(A) receptors. We have developed many novel steroid analogues in this effort. Recent work has resulted in synthesis of new enantiomer analogue pairs, novel ligands that probe various properties of the steroid pharmacophore, fluorescent neuroactive steroid analogues, and photoaffinity labels. Using these tools, combined with receptor binding and electrophysiological assays, we have begun to untangle the complexity of steroid actions at this important class of ligand-gated ion channel.

Anticonvulsant and anesthetic effects of a fluorescent neurosteroid analog activated by visible light

Most photoactivatable compounds suffer from the limitations of the ultraviolet wavelengths that are required for activation. We synthesized a neuroactive steroid analog with a fluorescent (7-nitro-2,1,3-benzoxadiazol-4-yl) amino (NBD) group in the beta configuration at the C2 position of (3alpha,5alpha)-3-hydroxypregnan-20-one (allopregnanolone, 3alpha5alphaP). Light wavelengths (480 nm) that excite compound fluorescence strongly potentiate GABAA receptor function. Potentiation is limited by photodepletion of the receptor-active species. Photopotentiation is long-lived and stereoselective and shows single-channel hallmarks similar to steroid potentiation. Other NBD-conjugated compounds also generate photopotentiation, albeit with lower potency. Thus, photopotentiation does not require a known ligand for neurosteroid potentiating sites on the GABAA receptor. Photoactivation of a membrane-impermeant, fluorescent steroid analog demonstrates that membrane localization is critical for activity. The photoactivatable steroid silences pathological spiking in cultured rat hippocampal neurons and anesthetizes tadpoles. Fluorescent steroids photoactivated by visible light may be useful for modulating GABAA receptor function in a spatiotemporally defined manner.

3Beta-hydroxysteroids and pregnenolone sulfate inhibit recombinant rat GABA(A) receptor through different channel property

3Beta-hydroxysteroids are pregnenolone sulfate-like GABA(A) receptor antagonists. The aim of the current study was to compare the functional differences between 3beta-hydroxysteroids and pregnenolone sulfate to inhibit GABA(A) receptors expressed in Xenopus oocytes. Recombinant rat GABA(A) receptors encoding wild type alpha1 beta2 gamma2L receptor, mutant alpha1V256S beta2 gamma2L and alpha1 beta2A252S gamma2L receptors were examined using a two-electrode voltage-clamp technique. A homologous mutation of the residue at 2'position closest to the cytoplasmic end of the M2 helix to serine on both alpha1 and beta2 subunit, alpha1V256S and beta2A252S, reduced the slow desensitization components of GABA-activated currents at saturating doses. Compared to the wild type receptor, the potency of GABA increased significantly in the alpha1V256S beta2 gamma2L receptor (P<0.05), whereas it decreased moderately in the alpha1 beta2A252S gamma2L receptor. We found that 5alpha-pregnan-3beta, 20(S)-diol (UC1019) and 5beta-pregnan-3beta, 20(R)-diol (UC1020) were the most effective blockers of maximal GABA responses among a panel of 3beta-hydroxysteroids. Pregnenolone sulfate, UC1019 and UC1020 were potent antagonists in the wild type receptor with calculated IC50s of 0.20+/-0.07 microM; 1.88+/-0.32 microM and 2.58+/-0.58 microM, respectively. The inhibitory effect of pregnenolone sulfate was significantly reduced in both mutants alpha1V256S beta2 gamma2L and alpha1 beta2A252S gamma2L receptors (P<0.05), whereas the inhibitory effects of UC1019 and UC1020 were reduced only in the mutant alpha1V256S beta2 gamma2L receptor. Pregnenolone sulfate promoted slow desensitization with prolonged GABA application in a dose-dependent manner in the wild type receptor, but not mutant receptors. On the contrary, UC1019 and UC1020 (< or = 20 microM) did not promote desensitization in both wild type and mutant receptors. In conclusion, the GABA(A) receptor inhibition by pregnenolone sulfate, but not 3beta-hydroxysteroids, was dependent on desensitization kinetics of the Cl- channels. A point mutation at M2 helix of the beta2-subunit (beta2A252S) can dramatically reduce the inhibitory effect of pregnenolone sulfate on the GABA(A) receptors without affecting the inhibitory properties of 3beta-hydroxysteroids. These results are consistent with the hypothesis that pregnenolone sulfate-inhibition does not share with 3beta-hydroxysteroids the coincident channel property at the GABA(A) receptor.

Cyclodextrins sequester neuroactive steroids and differentiate mechanisms that rate limit steroid actions

BACKGROUND AND PURPOSE

Neuroactive steroids are potent modulators of GABA(A) receptors and are thus of interest for their sedative, anxiolytic, anticonvulsant and anaesthetic properties. Cyclodextrins may be useful tools to manipulate neuroactive effects of steroids on GABA(A) receptors because cyclodextrins form inclusion complexes with at least some steroids that are active at the GABA(A) receptor, such as (3alpha,5alpha)-3-hydroxypregnan-20-one (3alpha5alphaP, allopregnanolone).

EXPERIMENTAL APPROACH

To assess the versatility of cyclodextrins as steroid modulators, we investigated interactions between gamma-cyclodextrin and neuroactive steroids of different structural classes.

KEY RESULTS

Both a bioassay based on electrophysiological assessment of GABA(A) receptor function and optical measurements of cellular accumulation of a fluorescent steroid analogue suggest that gamma-cyclodextrin sequesters steroids rather than directly influencing GABA(A) receptor function. Neither a 5beta-reduced A/B ring fusion nor a sulphate group at carbon 3 affected the presumed inclusion complex formation between steroid and gamma-cyclodextrin. Apparent dissociation constants for interactions between natural steroids and gamma-cyclodexrin ranged from 10-60 microM. Although gamma-cyclodextrin accommodates a range of natural and synthetic steroids, C(11) substitutions reduced inclusion complex formation. Using gamma-cyclodextrin to remove steroid not directly bound to GABA(A) receptors, we found that cellular retention of receptor-unbound steroid rate limits potentiation by 3alpha- hydroxysteroids but not inhibition by sulphated steroids.

CONCLUSIONS AND IMPLICATIONS

We conclude that gamma-cyclodextrins can be useful, albeit non-specific, tools for terminating the actions of multiple classes of naturally occurring neuroactive steroids.

Pregnenolone sulphate and Zn2+ inhibit recombinant rat GABA(A) receptor through different channel property

AIMS

We compared the antagonistic effects of state-dependent gamma-aminobutyric acid A (GABA(A)) receptor blockers picrotoxin, Zn(2+) and pregnenolone sulphate (PS) on GABA- and pentobarbital-activated currents in recombinant rat GABA(A) receptors in Xenopus oocytes.

METHODS

Experiments were performed with wild type rat alpha1 beta2 gamma2L and alpha1beta2 receptors, mutants alpha1V256S beta2 gamma2L and alpha1beta2A252Sgamma2L receptors by the two-electrode voltage-clamp technique.

RESULTS

In contrast to respective 3840- and 56-fold increases in Zn(2+) potencies to inhibit GABA- and pentobarbital-activated currents in the alpha1beta2 receptor, the corresponding potencies of PS remained unchanged in comparison with the alpha1 beta2 gamma2L receptor. A homologous mutation of the residue at 2' position closest to the cytoplasmic end of the M(2) helix to serine on both alpha1 and beta2 subunit, alpha1V256S and beta2A252S, abolished the inhibition of GABA(A) receptor by PS. In comparison with the wild type alpha1beta2gamma2L receptor, mutants alpha1V256S beta2 gamma2L and alpha1beta2 A252S gamma2L receptors did not affect the Zn(2+) inhibition. Furthermore, a significant increase in GABA potency was observed in the mutant alpha1V256S beta2 gamma2L receptor (P < 0.05), but not the mutant alpha1beta2 A252S gamma2L receptor compared with the wild type receptor.

CONCLUSIONS

Pregnenolone sulphate was a gamma2-subunit independent inhibitor in the GABA(A) receptor, whereas the Zn(2+) antagonism was profoundly influenced by the gamma2-subunit. It is likely that the 2' residue closest to the N-terminus of the protein at M(2) helix on both alpha1 and beta2 subunit are critical to the inhibitory actions of PS and the function of Cl(-) channels. These results are consistent with the hypothesis that PS behaves as a Cl(-) channel blocker that does not share with Zn(2+), the coincident channel property in the GABA(A) receptors.

Sulfated steroids as endogenous neuromodulators

Central nervous system function is critically dependent upon an exquisitely tuned balance between excitatory synaptic transmission, mediated primarily by glutamate, and inhibitory synaptic transmission, mediated primarily by GABA. Modulation of either excitation or inhibition would be expected to result in altered functionality of finely tuned synaptic pathways and global neural systems, leading to altered nervous system function. Administration of positive or negative modulators of ligand-gated ion channels has been used extensively and successfully in CNS therapeutics, particularly for the induction of sedation and treatment of anxiety, seizures, insomnia, and pain. Excessive activation of excitatory glutamate receptors, such as in cerebral ischemia, can result in neuronal damage via excitotoxic mechanisms. The discovery that neuroactive steroids exert rapid, direct effects upon the function of both excitatory and inhibitory neurotransmitter receptors has raised the possibility that endogenous neurosteroids may play a regulatory role in synaptic transmission by modulating the balance between excitatory and inhibitory neurotransmission. The sites to which neuroactive steroids bind may also serve as targets for the discovery of therapeutic neuromodulators.

Impact of the Hormonal Milieu on the Neurobiology of Alcohol Dependence and Withdrawal

Alcoholism, or alcohol dependence, is a complex disorder with withdrawal symptoms that are often problematic for those trying to recover from their dependence. As researchers attempt to elucidate the neurobiological underpinnings of alcohol dependence and withdrawal, it is becoming clear that numerous factors, including the hormonal environment, impact the manifestations of this disorder. Of particular interest is the observation that women have fewer and less severe withdrawal symptoms than do men even though they tend to suffer greater physiological harm from excessive alcohol consumption. In this article, the authors present an overview of their understanding of how gonadal and stress hormones interact with alcohol, which results in differential neurobiological responses between males and females. Thus far, data generated from representative animal models have shown significant differences between the sexes in behavioral responses and neuroadaptations to chronic alcohol consumption and withdrawal. Accumulating evidence suggests that treatment of alcoholism, including withdrawal, should be tailored to the patient's gender and hormonal status.

A spontaneous tonic chloride conductance in solitary glutamatergic hippocampal neurons

GABA-A receptors mediate both phasic synaptic inhibition and more recently appreciated tonic currents in the vertebrate central nervous system. We addressed discrepancies in the literature regarding the pharmacology of tonic currents by examining tonic currents in a controlled environment of dissociated, solitary glutamatergic neurons. We describe a novel tonically active, bicuculline-sensitive chloride conductance that is insensitive to gabazine and to picrotoxin and thus not mediated by conventional GABA receptors. We exclude a significant contribution from small conductance calcium-gated potassium (SK) channels. We also pharmacologically exclude calcium-gated chloride channels, glycine receptors and the chloride current associated with glutamate transport. Finally, we demonstrate that, although small, this current modulates neuronal excitability. We speculate that this tonic current may provide a complementary mechanism for the regulation of neuronal excitability, particularly in regions with low ambient GABA concentrations. We conclude that this bicuculline-sensitive conductance needs to be accounted for in studies of GABA tonic currents, lest it be confused with currents associated with GABA overflow.